The subject invention relates to the field of Nuclear Magnetic Resonance (NMR) imaging. The subject method and apparatus can allow an improved signal to noise ratio and is particularly advantageous for application to vertical field NMR imaging.
There are numerous examples of surface and volume coils described in the literature and available as commercial products. Several of these examples utilize multiple coils for an increased signal to noise ratio over a given field of view. In most cases, multiple coils examples have been applied to coil systems for use in a horizontal magnetic fields. Furthermore, the multiple coils are typically positioned to have no, or minimal, interaction with neighboring coils.
In order to image a large field of view, a first coil can be placed at one position, and one or more additional coils can be placed next to the first coil. If the coils interact with each other, it is preferable to switch the coils on and off such that only one coil is on at a time. Such a coil system can be referred to as a switchable coil. If the coils are positioned relative to one another such that they have minimal, or no, interaction, the coils can be switched on simultaneously allowing the entire field of view to be imaged at once. Such a coil system can be referred to as a phased array coil. The resultant image can have the signal to noise ratio of a small coil and the field of view of a large coil.
Advances in phased array coils have allowed linear coils to be positioned next to other linear coils, quadrature coils to be positioned next to other quadrature coils, and volume coils to be positioned next to other volume coils. In most cases, these coils have minimal mutual inductance and/or utilize some cancellation networks to reduce coupling. This concept can be applied to cover a larger area with several smaller coils.
U.S. Pat. No. 4,825,162 (Roemer et al.) discloses the use of multiple noninteracting coils to acquire an NMR image. In U.S. Pat. No. 4,825,162, the disclosed coils utilize simple linear designs that are intended to be used in a horizontal magnet system. These designs have no or minimal mutual inductance between the various coils, due to the relative position and/or the use of additional decoupling circuitry. However, while the goal of the Roemer device is to extend the field of view while preserving the signal to noise, the device finds limited application because two or more linear coils cannot be positioned to see the same field of view while preserving the signal to noise ratio. Also, since the minimization of mutual inductance is the first criteria for isolation, secondary methods are then used to improve the isolation further.
U.S. Pat. No. 5,394,087 (Molyneaux) discloses the use of quadrature coils positioned to minimize interaction between coils in order to achieve a higher signal to noise ratio than linear coils, while achieving a larger field of view compared to a single quadrature coil in horizontal field configurations. In U.S. Pat. No. 5,951,474 (Matsunaga et al.), the use of similar geometries to those disclosed in U.S. Pat. No. 5,951,474 is described for vertical field configurations. U.S. Pat. No. 5,258,717 discloses volume coils overlapped in the direction of the main field to extend the field of view, while preserving the signal to noise of a single volume coil for horizontal configurations. A major disadvantage of the configuration disclosed in the Molyneaux patent, the Matsunaga, et al. patent, and the Misic, et al. patent is the inability to use two or more linear coils positioned to see the same field of view while preserving the signal to noise where only one quadrature coil sees the same field of view. Also, the configurations disclosed in the Molyneaux and Misic et al. patents are designed to work primarily with horizontal fields. Although the Matsunaga et al. device is intended for use in vertical fields, considerable coupling can occur between adjacent quadrature coils, negatively impacting the signal to noise ratio.
U.S. Pat. No. 4,766,383 (Fox et al.) and U.S. Pat. No. 5,185,577 (Minemura) describe configurations utilizing crossed ellipse coils, such that two ellipsical coils are positioned to be orthogonal to one another for quadrature detection. The output is then sent to a quadrature combiner. A major disadvantage of the configurations disclosed in the Fox et al. and Minemura patents is that the crossed ellipse coils are used as quadrature coils, not array coils, and can substantially increase the signal to noise as compared to a solenoid.
U.S. Pat. No. 5,351,688 (Jones) describes the use of solenoids in a quadrature fashion, where one solenoid is used for the first direction of quadrature detection and a pair of solenoids are hooked together to make a Helmholtz pair in the second direction. Again, the output is sent to a quadrature combine. The major disadvantage of the configuration in the Jones patent is that the solenoid is used with a set of solenoids that are configured as a Helmholtz pair and then fed into a quadrature combine. This results in no increased field of view and no significant increase in signal to noise due to the addition of the Helmholtz coils, as the center of the Helmholtz coils is far away from the center of the single solenoid and the field sensitivity drops as the square of the distance away from the center of the loop.
U.S. Pat. No. 4,725,779 (Hyde, et al.) and U.S. Pat. No. 4,721,913 (Hyde, et al.) disclose the use of a single or multiple loop gap resonators forming linear coils. The loop gap resonators consist of opposite rotating current coils and planar pair coils. A significant disadvantage of the apparatus disclosed in U.S. Pat. No. 4,725,779 and U.S. Pat. No. 4,721,913 is the use of a single linear coil (either opposite rotating or planar pair) with reduced sensitivity over a solenoid coil. U.S. Pat. No. 4,866,387 (Hyde, et al.) discloses an opposite rotating current loop gap resonator and a planar pair which are combined to form a quadrature coil. U.S. Pat. No. 4,866,387 also discloses a plurality of planar pair and opposite rotating coils which are positioned adjacent to one another to form a network of coils. A drawback with respect to the configuration disclosed in U.S. Pat. No. 4,866,387 is the use of orthogonality for the isolation of overlapping and adjacent structures.